Spongy Brakes after hubs/pads Replacement - No air in system!

[D'Hag]

2003 Z-71 (LTZ) Tahoe

I recently replaced both front hubs, with new ABS sensors. I did this because the ABS light had come on, caused by the front sensors' not being able to read the rotors. Clearance on the sensors had increased due to hub bearing wear. (Funny, the hubs never made any sound, even though I could wiggle them side-to-side by hand.)

At the same time, I replaced the brake pads with new Performance Friction pads, and had the rotors turned. The caliper pistons moved freely. I cleaned and lubed all the mounts/pins/slides by the book.

Before I did these repairs, the brake pedal feel was firm and solid.

In doing these repairs, I did NOT open the brake hydraulic system at any point. The only "opening" I did was to take the cap off the master cylinder to check the fluid level, both before and after the work was done. It was over half full before starting. It was full after pressing the caliper pistons back.

Now the brake pedal is very spongy. It is possible to push the pedal hard enough to force the ABS system to activate, so it is possible to come to a safe stop. However, the sponginess in the pedal is very disturbing, especially after driving our other GM vehicles with normal pedal feel.

Even though there was no way air could have been introduced into the system, I have bled the brakes twice. (The "old fashioned" way, with someone working the pedal while I open/close the bleed fittings on the calipers. Working from farthest point to nearest point.) Both times, not a single bubble of air has been pushed out. One good thing-- The system now has all-new fluid in it.

No lights. No codes. No noises. Just a very spongy pedal.

Everything I read seems to point at the ABS system, and tells me to bleed the brakes. So what bit of ABS magic might I have missed? Is there some point where air won't be pushed out while bleeding? And if there is, how could air have gotten in there? No sponginess before the repair.

I've been wrenching on GM's pretty much my whole life (professionally as well as for myself), but this has me stumped. I'll check any possibility.

 

[clkelley]

I would fully flush the power steering fluid, especially when replacing the brake pads, rotors, etc. The power steering hydroboost system helps to supply added pressure to the brake system in general.

 

[clkelley]

Also, for future reference, there often is no need to replace the hubs when the abs sensors begin to act up. Check out the link below for instructions on how to do this in the future...

http://tahoeforum.com/showthread.php?t=526194

 

[D'Hag]

Also, for future reference, there often is no need to replace the hubs when the abs sensors begin to act up.

Oh, no... These hubs were bad, Bad, BAD! Definitely the cause of the ABS sensor code.

 

[D'Hag]

I would fully flush the power steering fluid, especially when replacing the brake pads, rotors, etc. The power steering hydroboost system helps to supply added pressure to the brake system in general.

Really??? I'll check it out. Time to buy the official GM manual for this puppy! I normally do for all my vehicles, anyway.

 

[clkelley]

Oh, no... These hubs were bad, Bad, BAD! Definitely the cause of the ABS sensor code.

I gotcha...lol

Really??? I'll check it out. Time to buy the official GM manual for this puppy! I normally do for all my vehicles, anyway.

Yes Sir, the brake master cylinder in trucks with a hydroboost system are able to maintain better braking pressure due to the hydroboost. That system utilizes power steering fluid to "actuate" the hydroboost unit itself.

When the power steering fluid gets old, the pedal will feel spongy, I have actually seen some where the fluid was so bad that you would just about have to push the pedal to the floor to get good braking response.

 

[Donut Slayer]

I wouldnt ever push the brake fluid back thru the master cylinder. It will mess up the ABS. I dont know the exact reason but every vehicle that I've seen with that done the ABS will throw a code and act funny. Best thing to do it crack the bleeder when re-seating the pistons back. Other than that, I dont have a fix for ya.

 

[D'Hag]

Finally was able to get back to it. Guess what. No hydroboost on my Tahoe. The only hose running to it is the vacuum line from the manifold, and the two brake lines that run to the ABS unit.
My son-in-law will be visiting soon. We'll bleed it to death....

 

[clkelley]

WOW, a regular brake booster on a 03 Z71!?!?! Man, sorry about that.

 

[D'Hag]

I wouldnt ever push the brake fluid back thru the master cylinder. It will mess up the ABS. I dont know the exact reason but every vehicle that I've seen with that done the ABS will throw a code and act funny. Best thing to do it crack the bleeder when re-seating the pistons back. Other than that, I dont have a fix for ya.

Apparently Donut Slayer is correct. Except that mine never threw a code. Just acted badly.

Anyway, I got it fixed.:waytogo: First, another thorough bleed of the system actually made it worse. I replaced the master cylinder, which got it back where it was before the bleed. I wasn't too bummed about doing that, as the truck has enough miles on it that a new master cylinder would not be out of line.

But the original problem still remained.

After a LOT of researching, I decided that the ABS system needed to be cycled. Not necessarily bled, as there should be no air in the system, but at least cycled in order to undo whatever I did to it when I back-pressed the caliper pistons.

After more digging, I discovered I had two choices:
I could take it to a dealer and have them "bleed" the ABS according to the shop manual's process using their Scan Tool. Our local dealers want a hundred bucks just to hook the Scan Tool up!:Whoa:

Other choice was to force the ABS to cycle itself, and hope that would reset it. It was worth a try, and if it didn't work there was still the dealer.

Following the instructions I found, I took the truck out to a loose-gravel road (just a quarter mile from home, out here in the countryside). Starting cautiously, I got up to a blistering 10 mph and stomped the brake pedal hard as I could. With all my strength, I was able to get just a half-dozen barks out of the ABS. But that was enough. The second time, the brakes applied a little harder and the ABS barked a little more. Each time, I could go a little faster and hit the brakes a little harder.

A couple dozen repeats of this, speed up to 50 mph near the end, was probably over-kill. But that fixed it!

I gave it one more bleed to be sure, and there was no air in the system. So after I made the mistake of pressing the fluid backwards through the system, I just needed to "reset" the ABS by forcing it to cycle itself. (I don't think the new master cylinder was a bad idea, either.)

So thanks, Donut Slayer! Your comment got me on the right track. And I'll never back-push the fluid in an ABS system again. Open the bleeder and press it out into a container.

 

[Donut Slayer]

:waytogo: glad you got it fixed.

 

[jrplz]

Apparently Donut Slayer is correct. Except that mine never threw a code. Just acted badly.

Anyway, I got it fixed.:waytogo: First, another thorough bleed of the system actually made it worse. I replaced the master cylinder, which got it back where it was before the bleed. I wasn't too bummed about doing that, as the truck has enough miles on it that a new master cylinder would not be out of line.

But the original problem still remained.

After a LOT of researching, I decided that the ABS system needed to be cycled. Not necessarily bled, as there should be no air in the system, but at least cycled in order to undo whatever I did to it when I back-pressed the caliper pistons.

After more digging, I discovered I had two choices:
I could take it to a dealer and have them "bleed" the ABS according to the shop manual's process using their Scan Tool. Our local dealers want a hundred bucks just to hook the Scan Tool up!:Whoa:

Other choice was to force the ABS to cycle itself, and hope that would reset it. It was worth a try, and if it didn't work there was still the dealer.

Following the instructions I found, I took the truck out to a loose-gravel road (just a quarter mile from home, out here in the countryside). Starting cautiously, I got up to a blistering 10 mph and stomped the brake pedal hard as I could. With all my strength, I was able to get just a half-dozen barks out of the ABS. But that was enough. The second time, the brakes applied a little harder and the ABS barked a little more. Each time, I could go a little faster and hit the brakes a little harder.

A couple dozen repeats of this, speed up to 50 mph near the end, was probably over-kill. But that fixed it!

I gave it one more bleed to be sure, and there was no air in the system. So after I made the mistake of pressing the fluid backwards through the system, I just needed to "reset" the ABS by forcing it to cycle itself. (I don't think the new master cylinder was a bad idea, either.)

So thanks, Donut Slayer! Your comment got me on the right track. And I'll never back-push the fluid in an ABS system again. Open the bleeder and press it out into a container.

This should be a sticky just save me a headache and a trip to the stealer ship ! Yes Yes Yes old but good write up ...............:waytogo:

 

[Droberts]

Thank you thank you thank you thank you!

I just replaced the brake master cylinder and my pedal was still spongy, just like before. I went out into the country and slammed my brakes several times, the problem is now fixed. You rock!